As a proton-conducting polymer electrolyte used in a solid polymer-type fuel cell, a perfluoroalkylsulfonic acid-based polymer electrolyte including Nafion (registered trademark of DuPont) has been mainly used since it is excellent in properties when used in a fuel cell. However, this electrolyte has a problem that a strength and heat resistance of a membrane thereof are low.
On the other hand, as a polymer electrolyte providing a membrane which is excellent in heat resistance and has a high membrane strength, for example, study of sulfonated polyether ketone (JP-A No. 11-502249) and sulfonated polyether sulfone (JP-A No. 10-45913 and JP-A No. 10-21943) in which a sulfonic acid group is introduced into aromatic polyether has been progressed. In these polymers, generally, as an introduced amount of a sulfonic acid group is larger, proton-conductivity grows higher, but at the same time, there is a possibility that water uptake of a polymer becomes high. And, a membrane made from a polymer having high water uptake has a problem that, when used in a fuel cell, a great dimensional change is caused by water produced during use of the fuel cell, and a strength thereof is reduced.
As a polymer for solving this problem, a block copolymer having one or more blocks in which a sulfonic acid group is introduced, and one or more blocks in which a sulfonic acid is not substantially introduced, and in which at least one block of all blocks has an aromatic ring on its main chain has been already proposed (JP-A No. 2001.250567).